Exhaust treatment system and method

ABSTRACT

An exhaust treatment system includes a catalytic device configured to receive an exhaust flow and an injector upstream of the catalytic device in an exhaust flow direction that injects a reductant into the exhaust flow. A controller is configured to determine a change in an amount of NOx and a change in an amount of the reductant downstream of the catalytic device due to a change in an amount of the reductant injected by the injector. The controller is configured to determine a slip factor corresponding to a relative weight of a NOx slip rate exiting the catalytic device compared to a reductant slip rate exiting the catalytic device, and determine a dosing command for providing to the injector based at least in part on the slip factor, and the change in the amount of NOx and the change in the amount of the reductant downstream of the catalytic device.

RELATED APPLICATION

This is application is a continuation-in-part of application No. 14/171,637 filed on Feb. 3, 2014, for “Exhaust Emission Prediction System and Method,” which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to an exhaust system, and more particularly, to an exhaust treatment system and method.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel powered engines, and other engines known in the art, may produce a flow of exhaust composed of gaseous and solid compounds, including particulate matter, nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent. The amount of one or more constituents of the flow of exhaust emitted from the engine may be regulated depending on the type, size, and/or class of engine.

One method that has been implemented by engine manufacturers to comply with the regulation of NOx exhausted to the environment is a strategy called selective catalytic reduction (SCR). SCR is a process by which gaseous or liquid reductant (e.g., a mixture of urea and water) is injected or dosed into the flow of exhaust from the engine. The combined flow may form ammonia (NH₃), which may then be absorbed onto an SCR catalyst. The ammonia may react with NOx in the flow of exhaust to form H₂O and N₂, thereby reducing the amount of NOx in the flow of exhaust.

The ability of the SCR catalyst to reduce NOx depends upon many factors, such as catalyst formulation, the size of the SCR catalyst, exhaust gas temperature, and urea dosing rate, With regard to the dosing rate, the NOx reduction efficiency tends to increase until the dosing rate reaches a certain limit. Above the limit, the NOx reduction efficiency may increase at a slower rate because the ammonia may be supplied at a faster rate than the NOx reduction process can consume. The excess ammonia, known as ammonia slip, may be expelled from the SCR catalyst.

It may be useful to control the amount of reductant that is dosed based on a known emission requirement or target, and based on a maximum allowed ammonia slip. One method of controlling the dosed amount of reductant is described in U.S. Pat. No. 8,034,291 (the '291 patent) issued to Qi et al. The '291 patent describes generating a dosing command that is between a lower limit that is based on the known emission requirement and an upper limit that is based on the maximum allowed ammonia slip. The system of the '291 patent controls the amount of reductant that is dosed so that the system performs between the limits corresponding to the known emission requirement and the maximum allowed ammonia slip. However, the system may not permit operation outside the strict limits.

The disclosed system is directed to overcoming one or more of the problems set forth above.

SUMMARY

In one aspect, the present disclosure is directed to an exhaust treatment system including a catalytic device configured to receive a flow of exhaust and an injector disposed upstream of the catalytic device in an exhaust flow direction. The injector is configured to inject a reductant into the flow of exhaust. The exhaust treatment system also includes a controller in communication with the injector, and the controller is configured to determine a change in an amount of NOx and a change in an amount of the reductant downstream of the catalytic device due to a change in an amount of the reductant injected by the injector. The controller is also configured to determine a slip factor corresponding to a relative weight of a NOx slip rate exiting the catalytic device compared to a reductant slip rate exiting the catalytic device, and determine a dosing command based at least in part on the slip factor, and the change in the amount of NOx and the change in the amount of the reductant downstream of the catalytic device. The controller is further configured to provide the dosing command to the injector.

In another aspect, the present disclosure is directed to a method of controlling an injection of a reductant into a flow of exhaust from an engine using a controller. The method includes injecting the reductant into the flow of exhaust with an injector disposed upstream from a catalytic device, and the injector and the catalytic device are disposed in an exhaust system for the engine. The method also includes passing the flow of exhaust through the catalytic device, and determining, using the controller, a first amount of NOx and a first amount of the reductant in the flow of exhaust at an exit of the catalytic device based at least in part on a first dosing rate of the reductant. The method further includes determining, using the controller, a second amount of NOx and a second amount of the reductant in the flow of exhaust at the exit of the catalytic device based at least in part on a second dosing rate of the reductant. The first dosing rate of the reductant is different from the second dosing rate of the reductant. In addition, the method includes determining, using the controller, a slip factor corresponding to a relative weight of a NOx slip rate exiting the catalytic device compared to a reductant slip rate exiting the catalytic device. The method further includes determining, using the controller, a dosing command based at least in part on the slip factor, the difference between the second amount of NOx and the first amount of NOx, and the difference between the second amount of the reductant and the first amount of the reductant. The method also includes providing, using the controller, the dosing command to the injector.

In another aspect, the present disclosure is directed to a non-transitory computer readable storage device storing instructions that are executable by at least one processor of a computer to cause the computer to perform a method for controlling an injection of reductant into a flow of exhaust from an engine. The method includes determining a change in an amount of NOx and a change in an amount of a reductant downstream of a catalytic device due to a change in an amount of the reductant injected by an injector disposed upstream of the catalytic device in an exhaust flow direction. The method also includes determining a slip factor corresponding to a relative weight of a NOx slip rate exiting the catalytic device compared to a reductant slip rate exiting the catalytic device, and determining a dosing command based at least in part on the slip factor, the change in the amount of NOx, and the change in the amount of the reductant downstream of the catalytic device. In addition, the method includes causing a change in the amount of the reductant injected by the injector based on the dosing command.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an engine and an exhaust emissions prediction and treatment system, according to an exemplary embodiment;

FIG. 2 is a graph showing ratios of amounts of NOx and ammonia downstream of the SCR catalyst to respective amounts of NOx and ammonia upstream of the SCR catalyst as a function of an ammonia to NOx ratio of the flow of exhaust upstream of the SCR catalyst, according to an exemplary embodiment;

FIG. 3 is a diagrammatic illustration of an SCR model for the exhaust emissions prediction and treatment system of FIG. 1;

FIG. 4 is a diagrammatic illustration of an engine and an exhaust emissions prediction and treatment system, according to another exemplary embodiment;

FIG. 5 is a diagrammatic illustration of the controller for the exhaust emissions prediction and treatment system of FIG. 4;

FIG. 6 is a diagrammatic illustration of an offset SCR model for the controller of FIG. 4;

FIG. 7 is a flow chart illustrating a method of predicting an emissions characteristic, according to an exemplary embodiment;

FIG. 8 is a flow chart illustrating a method of controlling the injection of the reductant, according to an exemplary embodiment; and

FIG. 9 is a graph showing slip factor as a function of a logarithm of a rolling average of a standard deviation of the inlet temperature of the SCR catalyst for different levels of average ammonia slip, according to an exemplary embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 1 is a diagrammatic illustration of a power source, such as an engine 10, of a machine and an exhaust emissions prediction and treatment system, according to an exemplary embodiment. The disclosed embodiment may be applicable to various types of machines such as, for example, a fixed or mobile machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, power generation, tree harvesting, forestry, or any other industry known in the art. The engine 10 may be an internal combustion engine, such as, for example, a diesel engine, a gasoline engine, a gaseous fuel-powered engine, or any other engine apparent to one skilled in the art. The engine 10 may alternatively be another source of power such as a furnace or any other suitable source of power for a powered system such as a factory or power plant. Operation of the engine 10 may produce power and a flow of exhaust. For example, each combustion chamber (not shown) of the engine 10 may mix fuel with air and burn the mixture therein to produce the flow of exhaust. The flow of exhaust may contain carbon monoxide, NOx, carbon dioxide, aldehydes, soot, oxygen, nitrogen, water vapor, and/or hydrocarbons.

An exhaust system 12 is provided with the engine 10 such that the flow of exhaust may be fluidly communicated from the engine 10 to the exhaust system 12. The flow of exhaust produced by the engine 10 may be directed from the engine 10 to components of the exhaust system 12 by flow lines. For example, as shown in FIG. 1, the flow lines may include pipes, tubing, conduits, and/or other exhaust-carrying structures known in the art through which the flow of exhaust may be directed to an injector 14 disposed upstream from a catalytic device, such as an SCR catalyst 16, in the exhaust system 12. Although not shown, other components such as, for example, one or more turbochargers or any other component known in the art for treating or handling exhaust may be disposed between the exhaust passageway of the engine 10 and the inlet of the exhaust system 12.

The injector 14 may be connected to a reductant supply (not shown) and may inject a reductant, such as urea, urea and water, ammonia, and/or other elements or compounds capable of chemically reducing compounds, e.g., NOx, contained within the flow of exhaust in the presence of, for example, catalyst materials. The injector 14 may include a nozzle (not shown) or other flow control device configured to assist in controllably releasing a flow of the reductant into the flow of exhaust from the engine 10. The nozzle may be any type of injector known in the art and may include any device capable of injecting and/or atomizing an injected fluid.

The SCR catalyst 16 may chemically reduce the amount of NOx in the flow of exhaust. The reductant injected into the flow of exhaust by the injector 14 upstream from the SCR catalyst 16 may be absorbed onto the SCR catalyst 16 so that the reductant may react with NOx in the flow of exhaust to form H₂O (water vapor) and N₂ (nitrogen gas). For example, a mixture of urea and water injected by the injector 14 may decompose to ammonia, and the SCR catalyst 16 may facilitate a reaction between the ammonia and NOx in the flow of exhaust to produce water and nitrogen gas, thereby removing NOx from the flow of exhaust. Thus, the reductant may be injected into the flow of exhaust as urea and water, and may enter the SCR catalyst 16 as ammonia. The SCR catalyst 16 may include catalyst materials such as, but not limited to, zeolites (e.g., iron zeolite or copper zeolite) or vanadia. After exiting the SCR catalyst 16, the flow of exhaust may be output from the exhaust system 12, e.g., released into the surrounding atmosphere, such as through a tail pipe.

FIG. 2 illustrates the operation of the SCR catalyst 16, according to an exemplary embodiment. The solid line shown in FIG. 2 indicates a ratio (NOx,_(RATIO)) of an amount of NOx exiting (or present downstream of) the SCR catalyst 16 (the NOx slip) to an amount of NOx entering (or present upstream of) the SCR catalyst 16 as a function of an ammonia to NOx ratio (ANR) of the flow of exhaust entering the SCR catalyst 16. The dashed line shown in FIG. 2 indicates a ratio (NH_(3,RATIO)) of an amount of ammonia exiting the SCR catalyst 16 (the ammonia slip) to an amount of ammonia entering the SCR catalyst 16 as a function of the ANR of the flow of exhaust entering the SCR catalyst 16. The shape of the curves for NOx,_(RATIO) and NH_(3,RATIO) may depend, for example, on the composition of the SCR catalyst 16 and on the operating conditions of the engine 10.

In an embodiment, NOx,_(RATIO), NH_(3,RATIO), and ANR may be determined using the following Equations (1)-(3):

NOx,_(RATIO)=NOx,_(D)/NOx,_(U)  (1)

NH_(3,RATIO)=NH_(3,D)/NH_(3,U)  (2)

ANR=NH_(3,U)/NOx,_(U)  (3)

where NOx,_(D) is the amount of NOx exiting the SCR catalyst 16, NOx,_(D) is the amount of NOx entering the SCR catalyst 16, NH_(3,D) is the amount of ammonia exiting the SCR catalyst 16, and NH_(3,U) is the amount of ammonia entering the SCR catalyst 16.

Thus, if NOx,_(RATIO) equals 1, then all of the NOx entering the SCR catalyst 16 exits the SCR catalyst 16, and if NOx,_(RATIO) equals 0, then none of the NOx entering the SCR catalyst 16 exits the SCR catalyst 16. Likewise, if NH_(3,RATIO) equals 1, then all of the ammonia entering the SCR catalyst 16 exits the SCR catalyst 16, and if NH_(3,RATIO) equals 0, then none of the ammonia entering the SCR catalyst 16 exits the SCR catalyst 16.

As shown in FIG. 2, there may be three general regimes in which the SCR catalyst 16 may operate. The regimes may correspond to three ranges of ANR such that regime I may span the lowest range of ANR (e.g., starting from zero), regime II may span an intermediate range of ANR between regimes I and III, and regime III may span the highest range of ANR. The locations of the boundaries separating the regimes shown in FIG. 2 may vary.

Regime I may correspond to an “ammonia starved” operation of the SCR catalyst 16. In this regime, the NH_(3,RATIO) of the SCR catalyst 16 may be close to or equal to zero, which indicates close to or equal to zero ammonia slip. The NOx,_(RATIO) of the SCR catalyst 16 may depend on the amount of ammonia available for the reduction of NOx such that increasing the dosing rate of the reductant (e.g., increasing the ANR) in this regime may lead to a decrease in the amount of NOx exiting the SCR catalyst 16 (e.g., a decrease in NOx,_(RATIO)). For example, there may be a linear or a substantially linear relationship between NOx,_(RATIO) and ANR in this regime. Thus, regime I may be characterized as producing relatively higher NOx slip and relatively lower ammonia slip exiting the SCR catalyst 16.

Regime III may correspond to an “ammonia rich” operation of the SCR catalyst 16. In this regime, the NOx,_(RATIO) of the SCR catalyst 16 may be close to or equal to zero, which indicates close to or equal to zero NOx slip. The NH_(3,RATIO) of the SCR catalyst 16 may depend on the dosing rate of the reductant such that increasing the dosing rate of the reductant (e.g., increasing the ANR) in this regime may lead to an increase in the amount of ammonia exiting the SCR catalyst 16 (e.g., an increase in NH_(3,RATIO)). For example, there may be a linear or a substantially linear relationship between NH_(3,RATIO) and ANR in this regime. Thus, regime III may be characterized as producing relatively higher ammonia slip and relatively lower NOx slip exiting the SCR catalyst 16,

In regime II, the slope of the NOx,_(RATIO) curve may be of approximately the same magnitude as the slope of the NH_(3,RATIO) curve, but the NOx,_(RATIO) curve may have a negative slope while the NH_(3,RATIO) curve may have a positive slope. Regime II may be characterized as producing relatively lower NOx slip than in regime I and relatively lower ammonia slip than in regime III. Therefore, depending on the application, it may be desirable to operate the SCR catalyst 16 in regime II.

In addition to the SCR catalyst 16, the exhaust system 12 may include one or more other aftertreatment devices configured to remove particulates and other constituents from the flow of exhaust, e.g., a filter for capturing particulates, ash, or other materials from the exhaust gas to prevent their discharge into the surrounding environment, such as a diesel particulate filter (DPF), a system for regenerating the filter by removing the particulate matter trapped by the filter, other catalytic devices, and/or other exhaust gas treatment devices. For example, a diesel oxidation catalyst (DOC) may be located upstream of the injector 14 and may raise the NO₂/NOx ratio, which may improve the NOx conversion efficiency of the SCR catalyst 16. An ammonia oxidation (AMOX) catalyst may be located downstream of the SCR catalyst 16 and may oxidize ammonia that slips from the SCR catalyst 16 to form N₂ and H₂O.

Referring back to FIG. 1, the exhaust emissions prediction and treatment system may include a controller 20 connected via communication lines 22 to one or more of the components of the engine 10 and the exhaust system 12. For example, the controller 20 may receive input via communication lines 22 from a variety of sources including, for example, a timer and/or one or more sensors configured to measure temperature, speed, pressure, fuel quantity consumed, flow rate, amount of the reductant injected, and/or other operating characteristics of the engine 10 and/or exhaust system 12. As shown in FIG. 1, the controller 20 may be connected by the communication lines 22 to an upstream NOx sensor 24 and a downstream NOx sensor 26. The upstream NOx sensor 24 may be located downstream of the engine 10, and upstream of the injector 14 and the SCR catalyst 16. The upstream NOx sensor 24 may also be located downstream from the turbocharger (if included) and/or upstream from the DOC (if included). Alternatively, the upstream NOx sensor 24 may be located at or near the outlet of the engine 10. The downstream NOx sensor 26 may be located downstream of the SCR catalyst 16, e.g., at or near the outlet of the SCR catalyst 16 and/or at or near the tail pipe (if included). The downstream NOx sensor 26 may also be located upstream or downstream of the AMOX catalyst (if included). Both of the NOx sensors 24, 26 may be physical (hardware) sensors that are cross sensitive to ammonia so that each NOx sensor 24, 26 is configured to generate a measured value that is indicative of a combination of NOx and ammonia concentrations at the location of the respective NOx sensor 24, 26.

The controller 20 may include components required to run an application such as, for example, a computer, a memory module, a secondary storage device (e.g., a database), and a processor or microprocessor, such as a central processing unit, as known in the art. The memory module may be configured to store information used by the processor, e.g., computer programs or code used by the processor to enable the processor to perform functions consistent with disclosed embodiments, e.g., the processes described in detail below. The controller 20 may be communicatively coupled with one or more components of the engine 10 and/or the exhaust system 12 to change the operation thereof. Optionally, the controller 20 may be integrated into the engine 10, e.g., as part of an engine control module (ECM). The controller 20 may use the inputs to form a control signal based on a pre-set control algorithm. The control signal may be transmitted from the controller 20 via the communication lines 22 to various actuation devices, such as one or more components of the engine 10 and/or the exhaust system 12, e.g., the injector 14 to control the timing and amount of injections.

The controller 20 may employ a model 30, such as a physics-based SCR model, that is based on one or more physical and/or chemical equations to estimate the performance (e.g., emissions characteristics) and other operating characteristics of the SCR catalyst 16. For example, the model 30 may include the following Equations (4)-(9) representing the reactions occurring at the SCR catalyst 16:

NH₃+S→NH₃*  (4)

NH₃*→NH₃+S  (5)

4NH₃*+4NO+O₂→4N₂+6H₂O  (6)

2NH₃*+NO+NO₂→2N₂+3H₂O  (7)

4NH₃*+3NO₂→3.5N₂+6H₂O  (8)

4NH₃*+3O₂→2N+6H₂O  (9)

Specifically, Equations (4) and (5) represent the adsorption and desorption of ammonia on the SCR catalyst 16, respectively. Equations (6) and (7) represent the standard and fast NOx reduction reactions in the SCR catalyst 16, respectively. Equation (8) represents the reduction reaction of NO₂ on the SCR catalyst 16, and Equation (9) represents the oxidation of ammonia on the SCR catalyst 16. The model 30 may also include other equations representing other reactions occurring at the SCR catalyst 16.

FIG. 3 shows a diagrammatic illustration of the model 30, according to an Embodiment. The model 30 may be used to estimate or predict one or more operating characteristics of the SCR catalyst 16 based on one or more inputs. As shown in FIG. 3, the inputs may include an inlet temperature 32 of the flow of exhaust entering the SCR catalyst 16, an exhaust mass flow rate 34 entering the SCR catalyst 16, an inlet NO concentration 36 of the flow of exhaust entering the SCR catalyst 16, an inlet NO₂ concentration 38 of the flow of exhaust entering the SCR catalyst 16, an inlet ammonia concentration 40 of the flow of exhaust entering the SCR catalyst 16, an inlet O₂ concentration 42 of the flow of exhaust entering the SCR catalyst 16, and/or an outlet pressure 44 of the flow of exhaust exiting the SCR catalyst 16.

The inputs may be estimated (e.g., using the controller 20 and/or virtual sensors) or measured (e.g., using physical sensors), as known in the art. For example, the inlet temperature 32 may be measured using a temperature sensor located upstream of the SCR catalyst 16, e.g., at or near the inlet of the SCR catalyst 16 and/or downstream from the DPF (if included). The exhaust mass flow rate 34 may be determined by the controller 20 based on one or more engine operating conditions, such as engine speed. The inlet NO and NO₂ concentrations 36, 38 may be determined by the controller 20 based on the measured value from the upstream NOx sensor 24 in addition to other parameters, such as the inlet temperature 32 and the exhaust mass flow rate 34. The inlet ammonia concentration 40 may be determined by the controller 20 based on the amount of the reductant that is dosed into the flow of exhaust, e.g., based on the command signal to the injector 14. The inlet O₂ concentration 42 may be determined by the controller 20 based on an estimated or measured amount of air intake into the engine 10 or based on a signal from an oxygen sensor in communication with the flow of exhaust. The outlet pressure 44 may be measured using a pressure sensor located downstream of the SCR catalyst 16, e.g., at or near the outlet of the SCR catalyst 16. For embodiments that include other exhaust treatment components (e.g., a DOC) upstream of the SCR catalyst 16, the controller 20 may be configured to estimate the inputs by taking into account the effect of the other exhaust treatment components on the composition of the flow of exhaust upstream of the SCR catalyst 16.

As shown in FIG. 3, the operating characteristics estimated by the model 30 may include one or more emissions characteristics, such as an outlet NOx concentration 46 (e.g., the combined concentration of NO and NO₂) of the flow of exhaust exiting the SCR catalyst 16 and/or an outlet ammonia concentration 48 of the flow of exhaust exiting the SCR catalyst 16. The model 30 may estimate the operating characteristics using Equations (4)-(9) and the inputs described above. Providing estimates for these emissions characteristics may be useful in determining the effectiveness of the SCR catalyst 16. Also, the controller 20 may be configured to use the estimates as feedback for controlling the dosing of the reductant using the injector 14. For embodiments that include other exhaust treatment components (e.g., an AMOX catalyst) downstream of the SCR catalyst 16, the controller 20 may be configured to estimate the operating characteristics by also taking into account the effect of the other exhaust treatment components on the composition of the flow of exhaust downstream of the SCR catalyst 16.

Because the downstream NOx sensor 26 is sensitive to both NOx and ammonia, the exhaust emissions prediction and treatment system may also take into account this cross-sensitivity in order to provide reliable and accurate estimates for the operating characteristics of the SCR catalyst 16. For example, the model 30 may determine a predicted value for the output from the downstream NOx sensor 26. If the predicted value is greater than the measured value, then the predictions of the outlet NOx and ammonia concentrations 46, 48 by the model 30 may be too high, which may indicate that the kinetics of the SCR catalyst 16 in the model 30 (the rates of the reactions represented using Equations (4)-(9)) are too slow. Because changing the kinetics in the model 30 may be difficult, the controller 20 may instead adjust one or more of the inputs to the model 30 to produce the same effect as changing the kinetics of the SCR catalyst 16 so that the model 30 may provide more accurate estimates for the operating characteristics. For example, in the embodiment shown in FIG. 3, the controller 20 may decrease the exhaust mass flow rate 34 to approximate the speeding up of the kinetics of the SCR catalyst 16. Specifically, the controller 20 may determine a correction factor 50 that may be used to adjust the exhaust mass flow rate 34 as described in more detail below.

On the other hand, if the predicted value is less than the measured value, then the predictions of the outlet NOx and ammonia concentrations 46, 48 by the model 30 may be too low, which may indicate that the kinetics of the SCR catalyst 16 in the model 30 are too fast The controller 20 may adjust the model 30 to slow down the kinetics of the SCR catalyst 16, e.g., by increasing the exhaust mass flow rate 34 using the correction factor 50. Thus, the controller 20 may adjust the exhaust mass flow rate 34 that is input into the model 30 using the correction factor 50 to adjust the kinetics of the SCR catalyst 16 represented in the model 30.

If the predicted value is approximately equal to the measured value, then the predictions of the outlet NOx and ammonia concentrations 46, 48 may be acceptable. Thus, in response, the controller 20 may not change the exhaust mass flow rate 34. Because the model 30 may be governed by Equations (4)-(9), which fix the relative quantities of the outlet NOx and ammonia concentrations 46, 48, the estimated values of the outlet NOx and ammonia concentrations 46, 48 may both be acceptable or not acceptable. The outlet NOx concentration 46 cannot be acceptable while the outlet ammonia concentration 48 is too low, or vice versa. Also, the outlet NOx concentration 46 cannot be too high while the outlet ammonia concentration 48 is too low, or vice versa.

FIG. 4 shows a diagrammatic illustration of the engine 10 and an exhaust emissions prediction and treatment system including a controller 120, according to another exemplary embodiment. Controller 120 may replace controller 20 in the exhaust emissions prediction and treatment system of FIG. 1 and may be connected via similar communication lines 22 to one or more of the components of the engine 10 and the exhaust system 12 as described above. As shown in FIG. 4, controller 120 may also be connected by one of the communication lines 22 to an upstream temperature sensor 122. The temperature sensor 122 may be located downstream of the injector 14 and upstream of the SCR catalyst 16, e.g., at the inlet of the SCR catalyst 16. Alternatively, the temperature sensor 122 may be located in or on the SCR catalyst 16, or upstream from the injector 14. The temperature sensor 122 may measure the inlet temperature 32 of the flow of exhaust entering the SCR catalyst 16 for inputting into the SCR model 30, as described above.

FIG. 5 shows a diagrammatic illustration of the controller 120. The controller 120 may employ one or more physics-based SCR models, such as an offset SCR model 130 and the SCR model 30 shown and described above in connection with FIG. 3. The offset SCR model 130 may be substantially similar to the SCR model 30 described above, except that one or more inputs to the offset SCR model 130 may be offset (e.g., increased or decreased), as described below, to predict the operation of the SCR catalyst 16. Thus, the SCR model 30 may be referred to as a “baseline” SCR model that estimates the “baseline” outlet NOx concentration 46 of the flow of exhaust exiting the SCR catalyst 16 and the “baseline” outlet ammonia concentration 48 of the flow of exhaust exiting the SCR catalyst 16.

FIG. 6 shows a diagrammatic illustration of the offset SCR model 130, according to an embodiment. Like the baseline SCR model 30, the offset SCR model 130 may be a physics-based SCR model, and may be based on one or more physical and/or chemical equations to estimate the performance (e.g., emissions characteristics) and other operating characteristics of the SCR catalyst 16. The offset SCR model 130 may include Equations (4)-(9) representing the reactions occurring at the SCR catalyst 16, as described above. As shown in FIG. 6, the offset SCR model 130 may be used to estimate or predict one or more operating characteristics of the SCR catalyst 16 based on one or more of the same inputs for the baseline SCR model 30.

The controller 120 may adjust one or more of the inputs using an offset 132 to predict the operation of the SCR catalyst 16. In an embodiment, as shown in FIG. 6, the offset 132 may be applied to the inlet ammonia concentration 40 to reflect an increase or decrease in the reductant injected by the injector 14, e.g., by a certain amount of ammonia (or ANR), or by a certain percentage of the injected ammonia (or ANR) (e.g., 5%, 10%, or more).

The operating characteristics estimated by the offset SCR model 130 may include one or more emissions characteristics, such as an offset outlet NOx concentration 136 (e.g., the combined concentration of NO and NO₂) of the flow of exhaust exiting the SCR catalyst 16 and/or an offset outlet ammonia concentration 138 of the flow of exhaust exiting the SCR catalyst 16. The offset SCR model 130 may estimate the operating characteristics using Equations (4)-(9) and the inputs (with one or more inputs adjusted using the offset 132) described above. Thus, the baseline SCR model 30 may determine the operating characteristics of the SCR catalyst 16 under current operating conditions, whereas the offset SCR model 130 may predict the operating characteristics of the SCR catalyst 16 under offset operating conditions, such as an increase in the dosed reductant.

Alternatively, instead of providing separate baseline and offset SCR models 30, 130, the controller 120 may be provided with a single SCR model. The single SCR model may determine the baseline outlet NOx concentration 46, the baseline outlet ammonia concentration 48, and other parameters associated with the operation of the SCR catalyst 16 under baseline operating conditions when receiving the inputs 32-44 adjusted using the correction factor 50. The single SCR model may also determine the offset outlet NOx concentration 136, the offset outlet ammonia concentration 138, and other parameters associated with the operation of the SCR catalyst 16 under offset operating conditions when receiving the inputs 32-44 adjusted using the offset 132 and the correction factor 50.

As another alternative, the correction factor 50 may be omitted from the baseline and offset SCR models 30, 130 (or the single SCR model replacing the separate baseline and offset SCR models 30, 130), e.g., if the SCR model(s) are determined to be sufficiently accurate without the correction factor 50 or if less accuracy is acceptable, depending on the application.

Referring hack to FIG. 5, the baseline outlet NOx concentration 46, the baseline outlet ammonia concentration 48, the offset outlet NOx concentration 136, and the offset outlet ammonia concentration 138 may be fed to a dosing control module 140 for determining a dosing command 142 to send to the injector 14 via communication line 22. The dosing control module 140 may determine the dosing command 142 at least in part on a slip factor that may indicate the relative weight of NOx slip compared to ammonia slip, as described below.

INDUSTRIAL APPLICABILITY

The disclosed exhaust emissions prediction and treatment systems may be applicable to any exhaust system. The exhaust emissions prediction and treatment systems may use the NOx sensors 24, 26 to adjust the model 30 to provide more accurate predictions for the emissions characteristics. The exhaust emissions prediction and treatment systems may also control the amount of the reductant injected by the injector 14 based on the slip factor indicating the relative weight of NOx slip compared to ammonia slip.

FIG. 7 shows a flow chart depicting an embodiment of an algorithm of the software control used in connection with the controller 20 and model 30 shown in FIGS. 1 and 3 for predicting an emissions characteristic. The steps described below may be repeated by the controller 20 periodically, e.g., every 0.12 seconds. Although the algorithm is described in connection with the controller 20, the algorithm may also be used in connection with the controller 120 shown in FIG. 4.

The controller 20 may determine the measured values of the NOx sensors 24, 26 (step 60). The controller 20 may also determine a predicted value for the output from the downstream NOx sensor 26 (step 62). The predicted value may be determined using the following Equation (10):

NOx′_(P2)=NOx+(0.8×NH₃)  (10)

where NOx′_(P2) is the predicted downstream NOx sensor value, NOx is the outlet NOx concentration 46 determined from the model 30, and NH₃ is the outlet ammonia concentration 48 determined from the model 30. The factor of 0.8 is dependent on the configuration of the downstream NOx sensor 26 and may vary depending on the downstream NOx sensor 26.

The controller 20 may then determine the correction factor 50 based on the measured values determined in step 60 and the predicted downstream NOx sensor value determined in step 62 (step 64). In an embodiment, the correction factor 50 may be calculated using the following Equation (11):

Correction Factor=ln(1−η₁)/ln(1−η₂)  (11)

where ln is the natural log function, η₁ is a conversion efficiency of the SCR catalyst 16 calculated using the measured values from the NOx sensors 24, 26, and η₂ is a conversion efficiency of the SCR catalyst 16 calculated using the predicted downstream NOx sensor value. In an embodiment, η₁ and η₂ may be determined using the following Equations (12) and (13):

η₁=(NOx′_(M1)−NOx′_(M2))/NOx′_(M1)  (12)

η₂=(NOx′_(M1)−NOx′_(P2))/NOx′_(M1)  (13)

where NOx′_(M1) is the measured value from the upstream NOx sensor 24, NOx′_(M2) is the measured value from the downstream NOx sensor 26, and NOx′_(P2) is the predicted downstream NOx sensor value determined using Equation (10). NOx′_(M1) may be used in Equation (13), instead of a predicted upstream NOx sensor value, based on the assumption that there is a negligible (if any) amount of ammonia at the location of the upstream NOx sensor 24.

The correction factor calculated using Equations (11)-(13) may equal approximately 1 when the predicted downstream NOx sensor value is approximately equal to the measured value. The correction factor may be less than 1 when the predicted downstream NOx sensor value is greater than the measured value. The correction factor may be greater than 1 when the predicted downstream NOx sensor value is less than the measured value.

The controller 20 may adjust the exhaust mass flow rate 34 using the correction factor 50 (step 66). In an embodiment, the exhaust mass flow rate 34 may be multiplied by the correction factor 50. Thus, if the correction factor is approximately 1 (the predicted downstream NOx sensor value is approximately equal to the measured value), then the exhaust mass flow rate 34 stays approximately the same. If the correction factor is less than 1 (the predicted downstream NOx sensor value is greater than the measured value), then the exhaust mass flow rate 34 is decreased. If the correction factor is greater than 1 (the predicted downstream NOx sensor value is less than the measured value), then the exhaust mass flow rate 34 is increased.

The controller 20 may input the adjusted exhaust mass flow rate 34 into the model 30 (step 68) and may determine the outlet NOx concentration 46 and the outlet ammonia concentration 48 using the model 30 with the adjusted exhaust mass flow rate 34 (step 70). Thus, using the adjusted exhaust mass flow rate 34 when there is an error between the predicted and measured values for the downstream NOx sensor 26 may have the effect of speeding up or slowing down the kinetics of the model 30.

The implementation of the correction factor may be understood using the relationship between the calculated correction factor 50, and the kinetic rate and space velocity of the model 30, which is shown in the following Equation (14):

Correction Factor=(k ₁ SV ₂)/(k ₂ SV ₁)  (14)

where k₁ is an actual global kinetic rate of the SCR catalyst 16; k₂ is a global kinetic rate associated with the model 30; SV₁ is an actual space velocity of the SCR catalyst 16; and SV₂ is an effective space velocity of the model 30, which depends on the physical dimensions of the SCR catalyst 16 and the exhaust mass flow rate 34. Changing the kinetic rate (k₂) associated with the model 30 using the correction factor may be difficult because it may involve changing the pre-exponential factors in Equations (4)-(9). In the embodiment described above, the exhaust mass flow rate is changed using the correction factor, which also changes the effective space velocity (SV₂) of the model 30 and produces the same effect as changing the kinetics (k₂) associated with the SCR model 300 The space velocity of the model 30 is relatively simple to change because it involves either changing the catalyst dimension, e.g., the length or diameter of the SCR catalyst 16 used in the model 30 or changing the exhaust mass flow rate 34, as described above. Alternatively, the correction factor may be used to adjust the length or diameter of the SCR catalyst 16 used in the model 30.

Optionally, the controller 20 may also check the calculations of the outlet NOx concentration 46 and the outlet ammonia concentration 48 (step 72). In an embodiment, the controller 20 may calculate the outlet NOx and ammonia concentrations using the following Equations (15) and (16):

NH_(3,P2)=(NOx′_(M2)+NH_(3,inj)−NOx′_(M1))/1.8  (15)

NOx,_(P2)=NOx′_(M2)−(0.8×NH_(3,P2))  (16)

where NOx,_(P2) is the outlet NOx concentration, NH_(3,P2) is the outlet ammonia concentration, NOx′_(M1) is the measured value from the upstream NOx sensor 24, NOx′_(M2) is the measured value from the downstream NOx sensor 26, and NH_(3,inj) is the inlet ammonia concentration 40. Equations (15) and (16) apply when the engine 10 is running in steady-state operation and do not account for oxidation of ammonia over the SCR catalyst 16 by oxygen in the flow of exhaust, which may occur at temperatures greater than 450 degrees Celsius, depending on the composition of the SCR catalyst 16. Therefore, this check may apply only during steady-state conditions and relatively lower temperatures.

During steady-state conditions and relatively lower temperatures, the controller 20 may calculate the outlet NOx and ammonia concentrations using Equations (15) and (16) and may compare the calculated values to the values determined in step 70 above. If the values determined in step 70 are outside a certain threshold of the values calculated using Equations (15) and (16) (e.g., within 10 or 20% of the values calculated using Equations (15) and (16)), then the controller 20 may notify an operator of an error. For example, there may be an error in the model 30 or in the calculation of the correction factor 50. Otherwise, if the values are within the acceptable threshold, then the controller 20 may return to step 60.

The flow chart described above in connection with FIG. 8 depicts an exemplary embodiment of the algorithm and software control. Those skilled in the art will recognize that similar algorithms and software control may be used without deviating from the scope of the present disclosure.

Several advantages over the prior art may be associated with the exhaust emissions prediction and treatment system described above. The exhaust emissions prediction and treatment system may take into account the cross-sensitivity to ammonia of the NOx sensors 24, 26 to adjust the model 30. Therefore, the model 30 may provide more accurate and reliable predictions of outlet NOx and ammonia concentrations downstream of the SCR catalyst 16, even when ammonia slip is occurring.

The exhaust emissions prediction and treatment system may be executed in real time, and is not limited to steady-state operating conditions and lower temperatures. By incorporating Equations (4)-(9) into the model 30, the model 30 may predict transient performance of the SCR catalyst 16 more accurately and may also be more accurate in higher temperature operation when there may be significant ammonia oxidation, unlike Equations (15) and (16). Equations (4)-(9) also allow the model 30 to estimate the kinetics of the reactions taking place in the SCR catalyst 16. The model 30 may also omit other reactions and limitations within the SCR catalyst 16 in order to be able to be executed in real time without sacrificing the relative accuracy of the estimates of the operating characteristics of the SCR catalyst 16. The model 30 may respond in real time to changes in the exhaust system 12, temporary deactivation or long term aging of the SCR catalyst 16, or even minor errors in the model 30, such as errors in dimensions of the SCR catalyst 16 input into the model 30.

The model 30 may help reduce the reliance on maps and look-up tables when predicting the performance of the SCR catalyst 16, which may make the controller 20 simpler to calibrate and implement for different engine platforms. Further, the model 30 in conjunction with the correction factor 50 may obviate the need to characterize engine-to-engine or exhaust system-to-exhaust system variations, and therefore reclaim operating margin that may have been previously allocated to account for these variations. The model 30 may also be used to help the controller 20 anticipate and mitigate the release of NOx downstream of the SCR catalyst 16 or ammonia slip events.

FIG. 8 shows a flow chart depicting an exemplary embodiment an algorithm of the software control used in connection with the controller 120 shown in FIG. 4 for controlling the injection of the reductant. The steps described below may be repeated by the controller 120 periodically, e.g., every 0.12 seconds.

The controller 120 may determine the correction factor 50 (step 150). In an embodiment, as described above in connection with the flow chart shown in FIG. 7, the controller 120 may determine the measured values of the NOx sensors 24, 26 (step 60), determine a predicted value for the output from the downstream NOx sensor 26 (step 62) using the baseline SCR model 30, and determine the correction factor 50 based on the measured values determined in step 60 and the predicted downstream NOx sensor value determined in step 62 (step 64). Optionally, step 150 may be omitted if the baseline and offset SCR models 30, 130 are sufficiently accurate without the correction factor 50 or if less accuracy is acceptable.

The controller 120 may determine the baseline outlet NOx concentration 46 and the baseline outlet ammonia concentration 48 using the baseline SCR model 30 (step 152). In an embodiment, as described above in connection with the flow chart shown in FIG. 7, the controller 120 may adjust the exhaust mass flow rate 34 input into the baseline SCR model 30 using the correction factor 50 (step 66), input the adjusted exhaust mass flow rate 34 into the baseline SCR model 30 (step 68), and determine the outlet NOx concentration 46 and the outlet ammonia concentration 48 using the baseline SCR model 30 with the adjusted exhaust mass flow rate 34 (step 70).

The controller 120 may also determine the offset outlet NOx concentration 136 and the offset outlet ammonia concentration 138 using the offset SCR model 130 (step 154). In an embodiment, the controller 120 may multiply the exhaust mass flow rate 34 input into the offset SCR model 130 by the same correction factor 50 applied in step 152. Also, the controller 120 may adjust the input ammonia concentration 40 based on the offset 132. The offset 132 may be stored in the controller 120, e.g., in the memory module or database. For example, the offset 132 may be an increase in the input ammonia concentration 40 by 10%. In addition, the controller 120 may copy other operating characteristics that are determined by the baseline SCR model 30 into the offset SCR model 130, such as an amount of ammonia that is stored by the SCR catalyst 16.

The controller 120 may then input the adjusted exhaust mass flow rate 34 and the adjusted input ammonia concentration 40 into the offset SCR model 130, and may determine the offset outlet NOx concentration 136 and the offset outlet ammonia concentration 138 using the offset SCR model 130. Thus, the controller 120 may predict the operating characteristics (e.g., the offset outlet NOx concentration 136 and the offset outlet ammonia concentration 138) of the SCR catalyst 16 using the offset SCR model 130 when the dosing of the reductant is increased by 10%.

Next, the dosing control module 140 may determine a first parameter P₁ and a second parameter P₂ based on the baseline outlet NOx concentration 46, the baseline outlet ammonia concentration 48, the offset outlet NOx concentration 136, and the offset outlet ammonia concentration 138 (step 156). In an embodiment, the first parameter P₁ and the second parameter P₂ may be determined using the following Equations (17) and (18):

P₁=(NOx,_(RATIO,OFFSET)−NOx,_(RATIO,BASELINE)/(ANR) _(OFFSET)−ANR_(BASELINE))  (17)

P₂=(NH_(3,RATIO,OFFSET)−NH_(3,RATIO,BASELINE)/(ANR) _(OFFSET)−ANR_(BASELINE))  (18)

where NOx,_(RATIO,BASELINE) is the NOx,_(RATIO) (the ratio of the amount of NOx exiting the SCR catalyst 16 to the amount of NOx entering the SCR catalyst 16) determined using Equation (1) by dividing the baseline outlet NOx concentration 46 from the baseline SCR model 30 by an inlet NOx concentration (e.g., determined from the upstream NOx sensor 24); NOx,_(RATIO,OFFSET) is the NOx,_(RATIO) determined using Equation (1) by dividing the offset outlet NOx concentration 136 from the offset SCR model 130 by the inlet NOx concentration; NH_(3,RATIO,BASELINE) is the NH_(3,RATIO) (the ratio of the amount of ammonia exiting the SCR catalyst 16 to the amount of ammonia entering the SCR catalyst 16) determined using Equation (2) by dividing the baseline outlet ammonia concentration 48 from the baseline SCR model 30 by an inlet ammonia concentration (e.g., determined based on the command signal to the injector 14); and NH_(3,RATIO,OFFSET) is the NH_(3,RATIO) determined using Equation (2) by dividing the offset outlet ammonia concentration 138 from the offset SCR model 130 by the inlet ammonia concentration 40 adjusted with the offset 132.

In other words, the first parameter P₁ may be determined by dividing a change in the amount of NOx downstream of the SCR catalyst 16 (e.g., corresponding to NOx,_(RATIO,OFFSET)−NOx,_(RATIO,BASELINE)) due to the offset 132 in the amount of the reductant injected by the injector 14 by a change in the amount of the reductant injected by the injector 14 (e.g., corresponding to ANR_(OFFSET)−ANR_(BASELINE)). The first parameter P₁ may correspond to a slope of the graph of NOx,_(RATIO) as shown in FIG. 2 and an approximation of a derivative of NOx,_(RATIO) with respect to ANR at current (baseline) conditions.

The second parameter P₂ may be determined by dividing a change in the amount of ammonia downstream of the SCR catalyst 16 (e.g., corresponding to NH_(3,RATIO,OFFSET)−NH_(3,RATIO,BASELINE)) due to the offset 132 in the amount of the reductant injected by the injector 14 by a change in the amount of the reductant injected by the injector 14 (e.g., corresponding to ANR_(OFFSET)−ANR_(BASELINE)). The second parameter P₂ may correspond to a slope of the graph of NH_(3,RATIO) shown in FIG. 2 and an approximation of a derivative of NH_(3,RATIO) with respect to ANR at current (baseline) conditions.

The dosing control module 140 may determine the slip factor based on a standard deviation of the temperature measured using the temperature sensor 122 (step 158). In an embodiment, the slip factor may indicate the relative weight of NOx slip compared to ammonia slip. For example, higher values of the slip factor may cause the controller to adjust the dosing of the reductant to operate the SCR catalyst 16 to reduce the NOx slip despite the risk of increased ammonia slip. Thus, the slip factor may function like a knob or dial for selecting a desired level of performance from the SCR catalyst 16. The dosing control module 140 may increase the slip factor when the dosing control module 140 determines that a lower NOx slip rate is desired and may decrease the slip factor when the dosing control module 140 determines that a lower ammonia slip rate is desired.

The dosing control module 140 may determine the slip factor based on at least one of user input, a look-up table, a map or graph, or a formula. For example, the dosing control module 140 may store, e.g., in the memory module or database, the look-up table, the map or graph, or the formula for determining the slip factor. The slip factor may depend on the inlet temperature of the flow of exhaust entering the SCR catalyst 16 measured using the temperature sensor 122 shown in FIG. 4 and/or a target ammonia slip level.

FIG. 9 shows a graph showing the slip factor as a function of a logarithm of a rolling average (e.g., a 60-second rolling average) of a standard deviation of the inlet temperature of the SCR catalyst 16 for producing different levels of average ammonia slip A, B, and C, according to an embodiment. The rolling average of the standard deviation of the inlet temperature may indicate how transient the operation of the SCR catalyst 16 is, which may indicate the likelihood of the occurrence of ammonia slip.

In an embodiment, the dosing control module 140 may store a map or graph that is similar to the graph shown in FIG. 9. The map includes one or more slip factor curves 170, 172, and 174 that depend on the logarithm of the rolling average of the standard deviation of the inlet temperature. Each slip factor curve 170, 172, and 174 corresponds to a different level of average ammonia slip A, B, and C, respectively. The number of different levels of average ammonia slip (the number of curves) may vary and is not limited to three. In an embodiment, the different levels of average ammonia slip may include, but are not limited to, e.g., 20, 40, and 60 parts per million (ppm).

The dosing control module 140 may determine a tolerable level of ammonia slip corresponding to one of the levels of average ammonia slip A, B, or C represented by the slip factor curves 170, 172, and 174. The selection may be made, e.g., based on user input or based on regulations. The dosing control module 140 may monitor the inlet temperature for the SCR catalyst 16 using the temperature sensor 122, determine the logarithm of the rolling average of the standard deviation of the inlet temperature, and determine the slip factor based on the determined logarithm using the slip factor curve 170, 172, or 174 corresponding to the selected tolerable level of ammonia slip. For example, if the dosing control module 140 determines that the tolerable level of ammonia slip is the average ammonia slip A, then the dosing control module 140 may use the corresponding slip factor curve 170 to determine the slip factor based on the determined logarithm of the rolling average of the standard deviation of the inlet temperature.

The slip factor curves 170, 172, and 174 may be determined using experimental data. To obtain the experimental data, the engine 10 may be run in one or more periods of operation having different operating conditions. For example, the engine 10 may be run at a relatively low transient period of operation (corresponding to a relatively low value of the logarithm of the rolling average of the standard deviation of the inlet temperature), a medium transient period of operation (corresponding to a medium value of the logarithm of the rolling average of the standard deviation of the inlet temperature), and a relatively high transient period of operation (corresponding to a relatively high value of the logarithm of the rolling average of the standard deviation of the inlet temperature). While the engine 10 is running, the slip factor may be varied, e.g., through trial and error, and the resulting average ammonia slip may be measured. The slip factors that produce the level of average ammonia slip A during the relatively low transient, medium transient, and relatively high transient periods of operation, respectively, may be determined and plotted as slip factor data points 180, as shown in FIG. 9. Likewise, the slip factors that produce the level of average ammonia slip B during the relatively low transient, medium transient, and relatively high transient periods of operation, respectively, may be determined and plotted as slip factor data points 182, and the slip factors that produce the level of average ammonia slip C during the relatively low transient, medium transient, and relatively high transient periods of operation, respectively, may be determined and plotted as slip factor data points 184. The slip factor curves 170, 172, and 174 may be determined. based on the slip factor data points 180, 182, and 184, respectively, using a curve fitting algorithm, as known in the art.

Alternatively, instead of using the map shown in FIG. 9, the dosing control module 140 may store the correlation between the logarithm of the rolling average of the standard deviation of the inlet temperature, the average ammonia slip level, and the slip factor in one or more look-up tables or formulas.

As another alternative, instead of determining the slip factor based on the inlet temperature, the dosing control module 140 may use a constant slip factor, which may be an average slip factor that is likely to produce ammonia slip and NOx slip levels that are below target thresholds, and/or the dosing control module 140 may receive user input specifying the slip factor. Using the constant slip factor may be acceptable, for example, when the machine is expected to be limited to applications that may result in ammonia slip and NOx slip levels that are below target thresholds using the constant slip factor.

Referring back to FIG. 8, after determining the slip factor, the dosing control module 140 may scale the first and second parameters P₁ and P₂ using the slip factor (step 160), in an embodiment, the dosing control module 140 may scale the first and second parameters by determining a net parameter P_(NET) using the following Equation (19):

P _(NET)=(P ₁ ×SF ₁)+(P ₂ ×SF ₂)  (19)

where SF₁/SF₂ is the slip factor determined in step 158, and the first and second parameters P₁ and P₂ are determined using Equations (17) and (18) described above. The values of SF₁ and SF₂ are used to scale the first and second parameters P₁ and P₂. Because the slip factor is the ratio of SF₁/SF₂, and not the individual values of SF₁ and SF₂, Equation (19) may be simplified to the following Equation (20), which assigns a value of 1 to SF₂ so that SF is equal to SF₁:

P _(NET)=(P ₁ ×SF)+(P₂)  (20)

where SF is the slip factor determined in step 158, and the first and second parameters P₁ and P₂ are determined using Equations (17) and (18) described above. Thus, the first and second parameters P₁ and P₂ may be scaled using the slip factor using either Equation (19) or Equation (20). The scaled first and second parameters P₁ and P₂ may then be added together to obtain the net parameter P_(NET).

The dosing control module 140 may determine or adjust the dosing command 142 based on the scaled first and second parameters P₁ and P₂ (step 162). In an embodiment, the dosing control module 140 may multiply the net parameter P_(NET), which is determined in step 160 based on the scaled first and second parameters P₁ and P₂, by the inlet NOx concentration (e.g., determined from the upstream NOx sensor 24) to determine the dosing command 142, e.g., the amount of the reductant to inject using the injector 14. After determining the amount of the reductant to inject, the dosing control module 140 may change the amount of the reductant all at once or may incrementally or gradually change the amount of the injected reductant to reach the determined amount after a period of time.

The dosing control module 140 may store, e.g., in the memory module or database, the slip factor so that when the engine 10 is shut down, the slip factor that is stored and was last used prior to shutdown may be used during the start-up of the engine 10 or after the engine 10 is started up, depending on when the dosing begins.

Several advantages over the prior art may be associated with the exhaust emissions prediction and treatment system. The exhaust emissions prediction and treatment system may take into account the relative weight of NOx slip compared to ammonia slip when determining the dosing rate for the reductant. The slip factors may be used to select the desired level of performance from the SCR catalyst 16. This desired level of performance may be adjusted on the fly while the engine 10 is operating, and therefore may allow greater control of the SCR catalyst 16 performance. The dosing control module 140 may determine the appropriate slip factor during operation given a desired target for ammonia slip while also keeping NOx slip relatively low.

Although the dosing control module 140 may not store information defining the boundaries between regimes I, II, and III, the dosing control module 140 may control the dosing to the SCR catalyst 16 so that the SCR catalyst 16 may operate generally in the area of the graph indicated as regime II in FIG. 2, depending on the slip factor that is used. Alternatively, it is possible to adjust the slip factor to allow the SCR catalyst 16 to operate close to or within the areas of the graph indicated as regime I or III. For example, when NOx slip reduction is a priority, but ammonia slip is not, the dosing control module 140 may adjust the slip factor to allow the SCR catalyst 16 to operate near or within the area of the graph indicated as regime III. On the other hand, when ammonia slip reduction is a priority, but NOx slip is not, the dosing control module 140 may adjust the slip factor to allow the SCR catalyst 16 to operate near or within the area of the graph indicated as regime I.

The dosing control module 140 may also adapt to different levels of transient operation by the engine 10. The dosing control module 140 may allow the SCR catalyst 16 to operate at or close to a desired target for ammonia slip without becoming unstable or deviating substantially from the target ammonia slip during transient operation of the engine 10. Because the controller 120 may rely on one or more physics-based models, the controller 120 may predict the operation of the SCR catalyst 16, rather than relying solely on sensors.

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed exhaust emissions prediction and treatment systems. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed exhaust emissions prediction and treatment systems. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents. 

What is claimed is:
 1. An exhaust treatment system comprising: a catalytic device configured to receive a flow of exhaust; an injector disposed upstream of the catalytic device in an exhaust flow direction, the injector being configured to inject a reductant into the flow of exhaust; a controller in communication with the injector, the controller being configured to: determine a change in an amount of NOx and a change in an amount of the reductant downstream of the catalytic device due to a change in an amount of the reductant injected by the injector; determine a slip factor corresponding to a relative weight of a NOx slip rate exiting the catalytic device compared to a reductant slip rate exiting the catalytic device; determine a dosing command based at least in part on the slip factor, and the change in the amount of NOx and the change in the amount of the reductant downstream of the catalytic device; and provide the dosing command to the injector.
 2. The exhaust treatment system of claim 1, wherein the controller is further configured to: determine a first parameter by dividing the change in the amount of NOx downstream of the catalytic device by the change in the amount of the reductant injected by the injector; determine a second parameter by dividing the change in the amount of the reductant downstream of the catalytic device by the change in the amount of the reductant injected by the injector; and determine the dosing command based on the first parameter and the second Parameter.
 3. The exhaust treatment system of claim 2, wherein the controller is further configured to: determine a change in a first ratio of an amount of NOx exiting the catalytic device to an amount of NOx upstream of the catalytic device to determine the change in the amount of NOx, wherein the first parameter corresponds to a first slope of the first ratio relative to an amount of the reductant injected by the injector; and determine a change in a second ratio of an amount of the reductant exiting the catalytic device to an amount of the reductant upstream of the catalytic device to determine the change in the amount of the reductant downstream of the catalytic device, wherein the second parameter corresponds to a second slope of the second ratio relative to an amount of the reductant injected by the injector.
 4. The exhaust treatment system of claim 2, wherein the controller is further configured to scale the first parameter and the second parameter using the slip factor, add the scaled first and second parameters to obtain a net parameter, and determine the dosing command using the net parameter.
 5. The exhaust treatment system of claim 4, wherein the controller is further configured to determine an amount of NOx upstream of the catalytic device and multiply the net parameter by the amount of NOx upstream of the catalytic device to determine the dosing command.
 6. The exhaust treatment system of claim 1, wherein the controller is further configured to determine the change in the amount of NOx and the change in the amount of the reductant downstream of the catalytic device using at least one physics-based model of the catalytic device.
 7. The exhaust treatment system of claim 6, wherein: the at least one physics-based model includes a baseline physics-based model and an offset physics-based model; and the controller is further configured to estimate a baseline amount of NOx and a baseline amount of the reductant exiting the catalytic device using the baseline physics-based model, and to predict an offset amount of NOx and an offset amount of the reductant exiting the catalytic device due to the change in the amount of injected reductant using the offset physics-based model.
 8. The exhaust treatment system of claim 7, wherein the controller is further configured to determine the change in the amount of NOx based on the baseline amount of NOx and the offset amount of NOx, and to determine the change in the amount of the reductant downstream of the catalytic device based on the baseline amount of the reductant and the offset amount of the reductant.
 9. The exhaust treatment system of claim 1, wherein the controller is further configured to determine the slip factor based on at least one of user input, a look-up table, a map, or a formula.
 10. The exhaust treatment system of claim 1, wherein the controller is further configured to increase the slip factor when the controller determines that a lower NOx slip rate is desired and decrease the slip factor when the controller determines that a lower reductant slip rate is desired.
 11. The exhaust treatment system of claim 1, wherein the controller is further configured to measure a temperature of the flow of exhaust and determine the slip factor based on the measured temperature.
 12. The exhaust treatment system of claim 11, wherein: the controller is in communication with a database storing a correlation between the slip factor and a standard deviation of the temperature based on a limit on the reductant slip rate; and the controller is further configured to determine the slip factor using the correlation.
 13. The exhaust treatment system of claim 1, further comprising: a database storing the slip factor after shutdown of the engine; wherein the controller is further configured to, during start-up of the engine, use the slip factor stored in the database after shutdown of the engine.
 14. The exhaust treatment system of claim 1, wherein the change in the amount of the injected reductant is an increase by at least 10%.
 15. The exhaust treatment system of claim 1, wherein the catalytic device is a selective catalytic reduction device.
 16. A method of controlling an injection of a reductant into a flow of exhaust from an engine using a controller, the method comprising: injecting the reductant into the flow of exhaust with an injector disposed upstream from a catalytic device, the injector and the catalytic device being disposed in an exhaust system for the engine; passing the flow of exhaust through the catalytic device; determining, using the controller, a first amount of NOx and a first amount of the reductant in the flow of exhaust at an exit of the catalytic device based at least in part on a first dosing rate of the reductant; determining, using the controller, a second amount of NOx and a second amount of the reductant in the flow of exhaust at the exit of the catalytic device based at least in part on a second dosing rate of the reductant, the first dosing rate of the reductant being different from the second dosing rate of the reductant; determining, using the controller, a slip factor corresponding to a relative weight of a NOx slip rate exiting the catalytic device compared to a reductant slip rate exiting the catalytic device; determining, using the controller, a dosing command based at least in part on the slip factor, the difference between the second amount of NOx and the first amount of NOx, and the difference between the second amount of the reductant and the first amount of the reductant; and providing, using the controller, the dosing command to the injector.
 17. The method of claim 16, further comprising: determining, using the controller, a first parameter by dividing the difference between the second amount of NOx and the first amount of NOx by the difference between the second dosing rate and the first dosing rate; and determining, using the controller, a second parameter by dividing the difference between the second amount of the reductant and the first amount of the reductant by the difference between the second dosing rate and the first dosing rate; wherein the dosing command is further determined based on the first and second parameters.
 18. A non-transitory computer readable storage device storing instructions that are executable by at least one processor of a computer to cause the computer to perform a method for controlling an injection of reductant into a flow of exhaust from an engine, the method comprising: determining a change in an amount of NOx and a change in an amount of a reductant downstream of a catalytic device due to a change in an amount of the reductant injected by an injector disposed upstream of the catalytic device in an exhaust flow direction; determining a slip factor corresponding to a relative weight of a NOx slip rate exiting the catalytic device compared to a reductant slip rate exiting the catalytic device; determining a dosing command based at least in part on the slip factor, the change in the amount of NOx, and the change in the amount of the reductant downstream of the catalytic device; and causing a change in the amount of the reductant injected by the injector based on the dosing command.
 19. The non-transitory computer readable storage device of claim 18, wherein the change in the amount of the injected reductant is a change from a first dosing rate to a second dosing rate, the first dosing rate of the reductant being different from the second dosing rate of the reductant, and the method further comprises: determining a first amount of NOx and a first amount of the reductant in the flow of exhaust downstream of the catalytic device based at least in part on the first dosing rate of the reductant; determining a second amount of NOx and a second amount of the reductant downstream of the catalytic device based at least in part on the second dosing rate of the reductant; determining the change in the amount of NOx as a difference between the second amount of NOx and the first amount of NOx; and determining the change in the amount of the reductant as a difference between the second amount of the reductant downstream of the catalytic device and the first amount of the reductant downstream of the catalytic device.
 20. The non-transitory computer readable storage device of claim 19, wherein the method further comprises: measuring a temperature of the flow of exhaust; and determining the slip factor based on the measured temperature. 